The data link layer, layer 2 of the seven-layer OSI (open systems interconnection) model of computer networking, is the protocol layer that transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. The data link layer provides the functional and procedural means to transfer data between network entities.
Unicast transmission, of unicast traffic, is the sending of data-link frames from a single source to a single network destination identified by a unique IP (internet protocol) address. Multi-destination transmission is the sending of data-link frames in a single transmission from a single source simultaneously to either: (i) a group of destination computers (multicast); or (ii) every device on the network (broadcast).
The spanning tree protocol (STP) is a network protocol that ensures a loop-free topology for any bridged ethernet local area network (LAN). The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. The spanning tree also allows a network design to include spare, or redundant, links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links. STP creates a spanning tree within a network of connected layer-2 bridges (typically ethernet switches), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes.
TRILL (transparent interconnection of lots of links) is an internet engineering task force (IETF) standard for enabling multipathing in a data center. The TRILL campus topology is implemented by devices called routing bridges (RBridges) or TRILL Switches. Routing bridges run a link state protocol amongst themselves. A link state protocol is one in which connectivity is broadcast to all the RBridges, so that each RBridge knows about all the other RBridges, and the connectivity between them. This gives RBridges enough information to calculate shortest path first (SPF) paths where an equal cost multipath (ECMP) routing strategy is available for unicast traffic, and calculate distribution trees for delivery of frames either to destinations whose location is unknown or to multi-destination groups.
A TRILL campus topology may support multiple topologies such that both unicast and multi-destination traffic is routed in the network. In a multi-topology network, the router uses logically different routing tables for different topologies.
ECMP is a routing strategy where next-hop frame, or packet, forwarding to a single destination can occur over multiple equal cost paths, or routes. Equal cost paths are those paths, or routes, that tie for top place in routing metric calculations.
The control plane is the part of the router architecture that is concerned with drawing the network map, or the information in a routing table that defines what to do with incoming frames. In most cases, the routing table contains a list of destination addresses and the outgoing node(s) associated with them.
The forwarding plane, sometimes called the data plane, defines the part of the router architecture that decides what to do with frames arriving on an inbound node. Most commonly, it refers to a routing table in which the router looks up the destination address of the incoming frame and retrieves the information necessary to determine the route from the receiving node, through the internal forwarding fabric of the router, and to the proper outgoing node(s).